The goal of our research is to unravel the molecular basis of the nuclear receptor actions using X-ray crystallography in conjunction with multidisciplinary molecular and biochemical approaches. The nuclear receptor retinoid X receptors (RXRs) are key ligand-activated transcriptional regulators involved in a wide range of human physiology, including development and metabolism. RXRs are well-established drug targets. However, the clinical use of RXR ligands is clearly tempered by side effects like toxicity, possibly associated with their low selectivity and affinity and also the cross-reactivity with other functions of RXRs. To understand the molecular basis of ligand-mediated signaling of RXRs in human physiology, we are planning to determine the crystal structures of a RXR multi-domain fragment bound to a DNA response fragment of their target genes. Following structure determination, we will perform mutational studies, biochemical analysis, and cell-based transcriptional assays to confirm functional significance of key structure elements. This study will reveal the molecular basis of the functional specificity of RXRs and its ligands at key regulatory steps: from ligand binding and to target gene recognition. The structural insights will provide a rational template for structure-based drug discovery of highly selective and efficient RXR ligands with reduced side effects for the therapeutic use in human disease. Moreover, the structure generated in this project will serve as a model to study the ligand-dependent regulation of other nuclear receptor family members. PUBLIC HEALTH RELEVANCE: This proposal addresses a critical need in the current nuclear receptor research by providing three-dimensional structures of the multi-domain retinoid X receptor bound to its target genes. The structural and functional studies will reveal detailed insights on how drug specificity and affinity can be further improved, thus providing a foundation for rational drug discovery for effective treatment of human disease like metabolic disease.